1. Technical Field
The present invention relates to a driving force control apparatus for a vehicle having a plurality of drive sources.
2. Related Art
Conventionally, in the field of electric vehicles or hybrid electric vehicles, vehicles that run by transmitting power from a plurality of drive sources to a plurality of wheels have been proposed. In such a vehicle, when required driving force or required torque is applied to the vehicle in response to a driving operation, there is a degree of freedom as to how power is distributed among the plurality of drive sources to obtain such driving force.
Japanese Unexamined Patent Application Publication (JP-A) No. 2015-136980, JP-A No. 2005-151691, JP-A No. 2014-217204, Japanese Patent No. 4765552, and JP-A No. 2009-159682 describe technologies that determine how to distribute the required driving force or required torque among a plurality of running motors. With the technologies described in JP-A No. 2015-136980, JP-A No. 2005-151691, JP-A No. 2014-217204, and Japanese Patent No. 4765552, the required torque or required driving force is distributed in a manner such that the overall energy efficiency increases. With the technology described in JP-A No. 2009-159682, the required torque is distributed taking into account driving stability and energy efficiency.
In a vehicle that runs by transmitting power from a plurality of drive sources to a plurality of wheels, driving stability changes depending on the ratio of driving force distributed to the plurality of wheels. Also, energy loss in drive sources and power transmission mechanisms differs depending on rotation speed and torque. Therefore, in such a vehicle, the driving force is preferably distributed among the plurality of wheels in a manner that improves both driving stability and energy efficiency.
The technology according to JP-A No. 2009-159682 aims to both improve energy efficiency of a plurality of drive sources, and improve driving stability. However, it is thought that the ideal driving force distribution that improves both energy efficiency and driving stability is not obtained with the technology described in JP-A No. 2009-159682.
For instance, with the driving force control apparatus described in JP-A No. 2009-159682, the required torque is distributed at a first distribution ratio that improves energy efficiency if driving is stable, and is distributed at a second distribution ratio that improves driving stability if driving is unstable (see paragraph and FIG. 4 in JP-A No. 2009-159682). However, with such control, hunting occurs in which the distribution ratio switches back and forth between the first distribution ratio and the second distribution ratio (see FIG. 4 in JP-A No. 2009-159682). When hunting occurs, the time that it takes for the distribution ratio of the required torque to transition between the first distribution ratio and the second distribution ratio increases, and as a result, energy efficiency and driving stability decrease.
Also, JP-A No. 2009-159682 also describes balance distribution control in which the driving force control apparatus distributes the required torque at an intermediate value between the first distribution ratio that improves energy efficiency and the second distribution ratio that improves driving stability (see paragraph and FIG. 8 in JP-A No. 2009-159682). However, energy efficiency does not change monotonically with a change in the distribution ratio of the required torque. Therefore, a balance between energy efficiency and driving stability is not achieved even when the distribution ratio is controlled to this intermediate value. In actuality, changing the distribution ratio of the required torque from the intermediate value to one of the first distribution ratio and the second distribution ratio often results in an improvement in both energy efficiency and driving stability.
Also, even when attempting to achieve both energy efficiency and driving stability, the inventors of the present invention thought it preferable to be able to select which of energy efficiency and driving stability to prioritize, and adjust the degree to which it is prioritized, in accordance with the running state. However, with the driving force control apparatus described in JP-A No. 2009-159682, priority is simply switched between energy efficiency and driving stability, depending on whether the same running state has continued for a certain period of time. Therefore, the degree of priority is not adjusted in accordance with various parameters indicative of the running state.